Recently there has been increased usage of fiber reinforced theremosetting resins for vehicle body parts. The general practice is to mold the body parts through use of a sheet molding compound (SMC) or a bulk molding compound (BMC). The SMC is a sheet composed of fibrous material, such as glass fiber matt or woven glass fabric, impregnated with an uncured or partially cured thermosetting resin system, such as a polyester resin system. At the time of use the SMC is cut to the desired size, placed in the mold and through the application of pressure and heat, the resin system is cured to form the article.
With the BMC, the glass fibers are in a chopped form and are mixed in bulk with the uncured resin system. At the time of use, a batch of the BMC is placed in the mold and through the application of heat and pressure the resin system is fully cured.
A problem inherent with the use of the SMC or BMC is that the molded article contains minute pits or voids due to shrinkage associated with the polymerization of the resin system at the elevated temperature. The pitted nature of the surface presents a problem when the article is to be painted to match other vehicle body components, for the painting accentuates the pitting. Because of this, it has been the practice to fill in the pits with a filler and sand the filled area prior to painting. However, the sanded area has a slightly different coloration after painting, so that it is generally necessary to sand the entire surface prior to painting to provide uniformity. Because of these problems and the hand labor required, vehicle body parts formed from SMC and BMC have been relatively expensive.
It has been recognized that it would be advantageous to apply a layer of a thermoplastic material to the outer surface of a thermosetting resin article in order to provide a smooth outer surface more receptive to painting, and yet achieve the high strength and rigidity associated with the thermosetting resin. However, attempts in the past to provide this type of composite structure has not been successful because an adequate bond has not been obtained between the thermoplastic material and the thermosetting resin base.